Throughout this application, various publications, patents, and published patent applications are referred to by an identifying citation. The disclosures of the publications, patents, and published patent applications referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.
As the rapid evolution of portable electronic devices continues, the need for safe, long-lasting, high capacity rechargeable batteries becomes increasingly evident. Under such circumstances, high energy density lithium secondary batteries are rapidly being developed that will eventually replace the conventional lead acid, nickel-cadmium, and nickel metal hydride batteries in many applications. In recent years, there has been considerable interest in developing high energy density cathode active materials and alkali metals as anode active materials for high energy density lithium secondary batteries to meet these needs.
Lithium and sulfur are highly desirable as the electrochemically active materials for the anode and cathode, respectively, of rechargeable or secondary battery cells because they provide nearly the highest energy density on a weight (2500 Wh/kg) or volume (2800 Wh/l) basis possible of any of the known combinations of materials. To obtain high energy densities, the lithium can be present as the pure metal, in an alloy or in an intercalated form, and the sulfur can be present as elemental sulfur or as a component in an organic or inorganic material with a high sulfur content, preferably greater than 50 weight percent sulfur.
Hereinafter, anodes containing the element lithium in any form are referred to as lithium-containing anodes. Cathodes containing the element sulfur in any form are hereinafter referred to as sulfur-containing cathodes.
Many battery systems comprising alkali metal containing anodes and sulfur-containing cathodes have been described. Exemplary of high temperature cells incorporating molten alkali metal anodes and molten sulfur cathodes separated by a solid electrolyte are those described in U.S. Pat. Nos. 3,993,503, 4,237,200 and 4,683,179. For operation, these storage cells must be heated to temperatures greater than about 320.degree. C. Of considerable recent interest are cells comprising alkali metal anodes and cathodes containing elemental sulfur that operate at considerably lower temperatures, particularly those with solid cathodes operating at ambient temperatures. Rechargeable lithium sulfur battery cells operating at room temperature have been described by Peled et al. in J. Power Sources, 1989, 26, 269-271, wherein the solid sulfur-containing cathodes are comprised of a porous carbon loaded with elemental sulfur. The nature of the porous carbon was not described, but cells constructed with these cathodes provided only a maximum of 50 cycles. The decline in capacity with cycling was attributed to loss of cathode active material.
U.S. Pat. No. 3,639,174 to Kegelman describes solid composite cathodes comprising elemental sulfur and a particulate electrical conductor. U.S. Pat. No. 4,303,748 to Armand et al. discloses solid composite cathodes containing an ionically conductive material together with elemental sulfur, transition metal salts, or other cathode active materials for use with lithium or other anodes in which, for example, the active sulfur or other cathode active material and the inert compounds with electrical conduction, such as graphite powder, are both particles of between 1 and 500 microns in diameter. Further examples of cathodes comprising elemental sulfur, an electrically conductive material and an ionically conductive material that operate in the temperature range from -40.degree. C. to 145.degree. C. are described in U.S. Pat. Nos. 5,523,179, 5,532,077, 5,582,623 and 5,686,201 to Chu. U.S. Pat. Nos. 5,552,244 and 5,506,072 to Griffin et al. describe metal-sulfur batteries using a cathode comprising a mixture of finely divided sulfur and graphite packed around a conductive electrode and covered with a porous separator. A minimum of 10 weight percent of graphite is needed to achieve sufficient conductivity in the cathode structure. No function other than providing conductivity is described for the graphite.
In spite of the many known systems, as for example described above, employing a solid cathode comprising elemental sulfur in rechargeable alkali metal sulfur battery systems has been problematic in obtaining good electrochemical efficiency and capacity, cycle life, and safety of the cells owing to the diffusion of sulfur active materials from the sulfur-containing cathode into the electrolyte and anode components. This has been particularly true in battery cells comprising a sulfur-containing cathode in combination with a lithium-containing anode. U.S. Pat. No. 3,907,591 to Lauck and an article by Yamin et al. in J. Power Sources, 1983, 9, 281-287 describe the reduction of elemental sulfur during the discharging of a lithium/sulfur cell to soluble lithium polysulfides in high concentrations in the electrolyte. Even partial reduction of the solid sulfur in the cathode forms polysulfides, such as lithium octasulfide, that are soluble in the organic electrolytes. In battery cells, these soluble polysulfides diffuse from the cathode into the surrounding electrolyte and may react with the lithium anode leading to its fast depletion. This leads to reduced capacity of the battery cell.
In attempts to reduce the problems associated with the generation of soluble polysulfides in alkali metal battery cells comprising elemental sulfur, battery cells have been developed utilizing cathodes comprised of sulfur-containing materials in which sulfur is chemically bound to an organic or carbon polymer backbone or to a low molecular weight organic compound. One such class of electroactive sulfur-containing materials has been referred to as organo-sulfur materials. Herein, the term "organo-sulfur materials" means a material containing organic sulfur compounds with only single or double carbon-sulfur bonds or sulfur-sulfur bonds forming disulfide (--S--S--) linkages.
U.S. Pat. Nos. 4,833,048 and 4,917,974 to Dejonghe et al. disclose liquid sulfur-containing cathodes comprising organo-sulfur materials of the formula, (R(S).sub.y).sub.n, where y=1 to 6; R is one or more different aliphatic or aromatic organic moieties having 1 to 20 carbon atoms; and n is greater than 1. U.S. Pat. No. 5,162,175 to Visco et al. describes the use of 1 to 20 weight percent of conductor particles, such as carbon black, in solid composite cathodes containing organo-sulfur materials having disulfide electroactive groups. These organo-sulfur materials undergo polymerization (dimerization) and de-polymerization (disulfide cleavage) upon the formation and breaking of the disulfide bonds. The de-polymerization which occurs during the discharging of the cell results in lower molecular weight polymeric and monomeric species, namely soluble anionic organic sulfides, which may dissolve into the electrolyte and cause self discharge, reduced capacity, and eventually complete cell failure, thereby severely reducing the utility of organo-sulfur materials as a cathode-active material in secondary batteries. Although the soluble discharge products are typically soluble organic sulfides rather than the inorganic polysulfides of the type formed with elemental sulfur, the detrimental effects on electrochemical efficiency and cycle life are similar. In addition, the organo-sulfur materials typically contain less than 50 weight percent of sulfur so they have a much lower energy density or theoretical specific capacity than elemental sulfur.
U.S. Pat. No. 5,324,599 to Oyama et al. discloses a solid composite cathode comprising a combination of a compound having a disulfide group and a conductive polymer, or an organo-disulfide derivative of a conductive polymer. In one variation, a complex is formed from a disulfide compound and a conductive polymer in a composite cathode layer so that the disulfide compound is not likely to leak out of the composite cathode into the electrolyte of the rechargeable battery.
In a similar approach to overcome the dissolution problem with organo-sulfur materials, U.S. Pat. No. 5,516,598 to Visco et al. discloses solid composite cathodes comprising metal/organo-sulfur charge transfer materials with one or more metal-sulfur bonds, wherein the oxidation sate of the metal is changed in charging and discharging the positive electrode or cathode. The metal ion provides high electrical conductivity to the cathode, although it significantly lowers the cathode energy density and capacity per unit weight of the polymeric organo-sulfur material. There is no mention of retarding the transport of soluble reduced sulfide or thiolate anion species formed during charging or discharging of the cell.
Another class of electroactive sulfur-containing materials comprises carbon-sulfur polymer materials, for example, as described in U.S. Pat. Nos. 5,529,860, 5,601,947 and 5,609,702, and in copending U.S. patent application Ser. No. 08/602,323 to Skotheim et al. These references also describe the use of conductive carbons and graphites, conductive polymers, and metal fibers, powders, and flakes as conductive fillers with carbon-sulfur polymer materials. Herein, the term "carbon-sulfur polymer materials" means materials comprising carbon-sulfur polymers with carbon-sulfur single bonds and with sulfur-sulfur bonds comprising trisulfide (--S--S--S--), tetrasulfide (--S--S--S--S--), or higher polysulfide linkages. The carbon-sulfur polymer materials comprise, in their oxidized state, a polysulfide moiety of the formula, --S.sub.m --, wherein m is an integer equal to 3 or greater.
Several approaches have been described to inhibit or retard the transport or diffusion of soluble polysulfides from the cathode to the electrolyte. U.S. Pat. No. 3,806,369 to Dey et al. describes an ion exchange membrane between the cathode and the electrolyte/separator layer to inhibit the passage of polysulfides or other anions from the cathode into the electrolyte. Without this barrier layer, the soluble polysulfides or other anions form insoluble films on the cathode and shorten the cycle life of the cell. U.S. Pat. No. 3,532,543 to Nole et al. describes the attempt to use copper halide salts to limit the formation of polysulfides in a solid cathode containing elemental sulfur. U.S. patent application Ser. No. 08/859,996, titled "Novel Composite Cathodes, Electrochemical Cells Comprising Novel Composite Cathodes, and Processes for Fabricating Same" to the common assignee, discloses the addition of a class of electroactive transition metal chalcogenide materials to sulfur-containing cathodes to encapsulate or entrap the sulfur-containing materials to retard the transport of soluble polysulfides and sulfides from the cathode into the electrolyte.
Barrier layers, as for example those described heretofore, can be effective in preventing excessive diffusion of soluble cathode reduction products, such as inorganic polysulfides, into the electrolyte, thereby improving cycle life and safety from the levels obtained when excessive inorganic polysulfides and other soluble cathode reduction products are present in the electrolyte. However, these barrier layers may have disadvantages. Besides the cost and the non-cathode active volume occupied by the materials, they may effectively block the transport of desirable soluble or insoluble anionic species into the electrolyte. Also, the barrier layer may be only partially effective so that there is a slow buildup of soluble cathode reduction products in the electrolyte. While low concentrations of polysulfides initially may be acceptable in the early cycles of the cell, in the later charge-discharge cycles of the cell, the concentrations of the soluble polysulfides and other anions may become too high or excessive, thereby shortening the cycle life and decreasing cell safety.
Japanese Patent Publication No. 09-147868, published Jun. 6, 1997, describes the use of active carbon fibers to absorb electroactive sulfur-containing materials in cathodes of secondary batteries and to provide increased cycle life at high discharge currents. These active carbon fibers are characterized by highly microporous structures with specific surface areas above 1000 m.sup.2 /g, which absorb large amounts of sulfur-containing materials such as 30 to 50 weight percent, into the pores. These active carbon fibers also have diameters greater than 1 micron, typically in the range of 2 to 6 microns.
Despite the various approaches proposed for the fabrication of high energy density rechargeable cells comprising elemental sulfur, organo-sulfur or carbon-sulfur polymer materials in a solid cathode, there remains a need for materials and cell designs that prevent the excessive out-diffusion of sulfides and polysulfides from the cathode layers in these cells, improve the electrochemical utilization of cathode active materials and cell efficiencies, and provide safe rechargeable cells with high rates and capacities over many cycles.